Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization, pumping an atomic laser and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution by adjusting the phase, with a temporal resolution of 3as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity

The coupling of two autoionizing states or of a discrete and an autoionizing state by a strong laser field is studied analytically as well as numerically. The motion of the complex energies is traced as a function of the field strength for different field frequencies and atomic parameters. Most interesting is the critical region where a crossing (or an avoided crossing) of the trajectories occurs. At this critical field intensity, level repulsion in the complex plane occurs. With further increasing intensity, the complex energies move differently. When the resonances are coupled mainly via one common continuum, resonance trapping dominates, i.e. a short- and a long-lived resonance state are formed (level repulsion along the imaginary axis). When, however, the direct coupling dominates, level repulsion along the real axis takes place. Population trapping (defined by a vanishing decay width of one of the states at finite intensity) results from the interplay of the direct coupling of the states and their coupling via the continuum. We also studied the corresponding variation of the cross section for ionization of a laser-driven atom by the probe field.

Double poles of the S-matrix and avoided level crossings are related to branch points in the complex plane. Although the number of these branch points is very small, they determine the dynamics of the system in a decisive manner. We study, in the adiabatic regime, the effects induced by two strong laser fields in the continuum of the H atom. In the photoabsorption spectra of the probe field from the ground 1s state, the coupling to the 2s, 5s, 5d and 5g states is considered. The quasi-level avoided crossing in the laser-induced continuum structures is traced theoretically as a function of the intensities of the strong laser fields. Under certain conditions, the quasi-levels cross and the S-matrix has a double pole. The relation of the laser-induced continuum structures to branch points in the complex plane is discussed.

Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field. DOI: 10.1103/PhysRevLett.101.193002 PACS numbers: 32.80.Fb, 32.80.Rm Multiphoton single-color ionization in intense optical or infrared laser fields has been the subject of multiple experimental and theoretical studies for more than two decades and is by now a very well understood process (e.g., [1]). The extension of these studies to multiphoton absorption in the photoionization continuum was followed by the discovery that high order harmonics of the fundamental laser frequency are emitted in the extreme ultraviolet (XUV) when a strong femtosecond optical laser pulse interacts with a gas jet (e.g., [2,3]). The combination of different wavelengths, one in the XUV and the other in the visible or near infrared, opens new opportunities. It has recently permitted the investigation of above-threshold ionization (ATI) as the result of the combined interaction of both fields [4][5][6]. In this case the dominant contribution comes from processes in the course of which the emitted electron exchanges photons with the dressing laser field via stimulated emission (or absorption) resulting in a comb of sidebands disposed on both sides of the main photoelectron line.Theoretical studies have established that the sideband intensity depends on the electron kinetic energy as well as on the strength and polarization state of the optical laser field [7]. Fitting theoretical profiles to the measured sideband signals should yield the main parameters which govern the photon-atom interaction in this regime. For example, changing the polarization of either of the radiation beams gives rise to ''dichroic effects'' in the photoelectron spectrum. It therefore opens the possibility to control the relative contributions of photoionization channels with different angular momenta.This approach has been extensively used in studies of atomic ionization by weak monochromatic radiation from synchrotrons and continuous lasers, where at least one resonant intermediate state is involved, and the basic photon-electron interaction is completely dominated by this resonant excitation [8]. The use of high harmonic XUV sources to generate similar processes in the nonresonant continuum is complicated by very difficult analysis, since contributions from several harmonics and their mutual interferenc...

The line shape of resonances in the overlapping regime is studied by using the eigenvalues and eigenfunctions of the effective Hamiltonian of an open quantum system. A generalized expressionq k (E) for the Fano parameter of the resonance state k is derived that contains the interaction of the state k with neighboured states l = k via the continuum. It is energy dependent since the coupling coefficients between the state k and the continuum show a resonance-like behaviour at the energies of the neighboured states l = k. Under certain conditions, the energy dependentq k (E) are equivalent to the generalized complex energy independent Fano parameters that are introduced by Kobayashi et al. in analyzing experimental data. Long-lived states appear mostly isolated from one another in the cross section, also when they are overlapped by short-lived resonance states. Theq k (E) of narrow resonances allow therefore to study the complicated interplay between different time scales in the regime of overlapping resonance states by controlling them as a function of an external parameter.Typeset using REVT E X 1